The present invention relates to a superconducting substrate structure comprising a high temperature superconducting ground plane for epitaxial growth of multilayers thereon. The insertion also relates to a multilayer structure comprising a number of alternating high temperature superconducting film (HTS) and insulating layers epitaxially grown on a substrate structure. Still further the invention relates to a method of producing a substrate structure with a superconducting ground plane and a method of producing a multilayer structure.
The field of superconducting electronics has become more and more attractive and undergoes a fast development. There is a need for superconducting digital and microwave circuit devices which cart be made as small as possible, very compact and which, in addition thereto, are fast. Superconducting digital and microwave devices require multilayer structures which consist of insulating and superconducting layers stacked on top of one another. For example superconducting microwave circuits require a superconducting ground plane to confine the fields in order to reduce cross-talk between two adjacent lines. In so called Rapid Single Flux Quantum digital circuits (RSFQ) cf. for example xe2x80x9cRSFQ Logic/Memory Family: A New Josephson-Junction Technology for Sub-Teraherz Clock Frequency Digital Systemsxe2x80x9d by K. K. Likharev and V. K. Semenov in IEEE Transaction on Applied Superconductivity, Vol. 1, No. 1. March 1991 which presents a RSFQ circuit family which is based on processing of single flux quanta wherein each digital information bit is represented by a single flux quantum (a fluxon), also needs a superconducting ground plane in order to reduce he circuit inductance to such an extent that it get possible to build complex circuits.
An article by A. L. Vasiliev et al., in Physica C, 244, 373-378 (1995) discloses the making of a superconductor ground plane in HTS circuits by growing an insulating film on top of an HTS-film which is epitaxially grown on an insulator. However, a serious disadvantage of this arrangement is that the top insulating layer will not be smooth enough for multilayers to grow epitaxially thereon. This actually constitutes one of the main obstacles to the development of complex superconducting digital circuits and microwave circuits.
WO 94/14202 shows an electrical interconnect structure which comprises a number of high temperature superconducting signal layer substructures Such a structure may comprise signal layer substructures bonded by dielectric layers bonded to one another and to a base substructure containing power and ground planes. This document among others discusses the problems involved in sequential epitaxial deposition of multilayers, among others that the materials have to be chosen such that the coefficients of thermal expansion of all the materials are substantially the same and that the dielectric materials that are used also have to have the proper-electric constant and low microwave losses. The document among others discloses processes for making structures in which no all layers involve epitaxial deposition thus avoiding the severe constraints. However, the structures illustrated in this document and the method of making such are comparatively complicated and do also not provide an entirely satisfactory structure.
What is needed is therefore a superconducting substrate structure having a superconducting ground plane through which it is possible to build complex electronic circuits, in particular complex superconducting digital microwave circuits. Particularly a substrate structure is needed through which the growing of multilayers structures thereon, particularly through epitaxial growth, is enabled. Moreover a substrate structure is needed through which an insulating layer is provided which is smooth enough for multilayers to be grown epitaxially on top of it. Particularly a substrate structure is needed which comprises a large area substrate having an optically smooth surface suitable for epitaxial growth on it and which has a buried high temperature superconducting ground plane.
A multilayer structure having a buried HTS film serving as a ground plane is also needed fulfilling the above mentioned objects and which electronic circuits can be etched out of. Particularly a multilayer structure formed by epitaxial growth is needed fulfilling the above mentioned objects.
A method of producing a superconducting substrate structure as well as a multilayer structure is also needed through which the above mentioned objects are fulfilled and which is simple to implement and which moreover does not involve any high costs.
Therefore a superconducting substrate structure comprising a high temperature superconducting ground plane e.g. for epitaxial growth of multilayers thereon is provided which comprises a composite substrate structure comprising a first substrate layer aid a second substrate layer. Each of the substrate layers are covered by a HTS film on at least one side thereof which HTS films are bonded together through annealing to form a buried superconducting layer and of which one of the substrate layers is polished to form a smooth insulator adjacent to an HTS layer In particular the first and the second substrate layers, before bonding of the superconducting films arranged thereon, have a thickness of about 0.1-1 mm. In a particular implementation the high temperature superconducting films covering said first and second substrate layers which subsequently are bonded together forming a HTS layer each have a thickness of about 200 nm. Advantageously the substrate structure comprises a large area substrate structure. In an advantageous implementation a multilayer structure is epitaxially grown on top of the substrate layer which forms a smooth insulator. Particularly the HTS layer is formed in that the high temperature superconducting films are bonded to each other through annealing in oxygen under a high pressure and at a high temperature. Particularly the polished substrate layer forms a smooth insulator having a thickness of about 1-50 xcexcm. In particularly advantageous embodiment the HTS films comprise YBCO and the substrate layers advantageously comprise SrTiO3, MgO, LaAlO3, Yxe2x80x94ZrOz, other alternatives however also being possible.
Therefore also a multilayer structure is provided which comprises a number of alternating high temperature superconducting layers and insulating layers epitaxially grown on a substrate structure comprising a superconducting ground plane. The substrate structure comprises a composite substrate structure comprising a first substrate layer and a second substrate layer, each covered by a HTS film on at least one side thereof respectively, which HTS films are bonded together through annealing to form a buried. HTS layer. The substrate layer on which the HTS films and insulting layers are grown is polished to form a smooth insulator, thus suitable for epitaxial growth thereon. Particularly the substrate layer which is not polished, i.e. the second substrate layer, has a thickness of about 0.1-1 mm whereas the HTS layer is made of films having a thickness of about 200 nm each whereas the polished first substrate layer is polished to have a thickness of about 1-50 xcexcm (although it initially may have had the same thickness as the other substrate layer). Particularly the high temperature superconducting films of the substrate structure are bonded to each other through annealing in oxygen under a high pressure and at a high temperature. The HTS films particularly comprise YBCO whereas the substrate layers comprise SrTiO3, LaAlO3, MgO or Yxe2x88x92Zro2. 
Therefore also a use of a multilayer structure as discussed above is disclosed for making superconducting microwave circuits through etching in the structure, in which the HTS layer forms a ground plane.
Therefore also a method of producing a substrate structure with a superconducting ground plane is disclosed. The method comprises the steps of providing a first and a second substrate layer covering each of said first and second substrate layers by a thin HTS film respectively; arranging one of the substrate layers so in relation to the other that the thin HTS films get in lose contact; bonding the HTS films together trough annealing in oxygen at a high temperature and under a high pressure to form a HTS layer; polishing one of the substrate layers on that side of it that is opposite to the HTS-layer to form a smooth insulating layer. Particularly the substrate layers initially have a thickness of about 0.1-1 mm. Particularly one of the substrate layers is polished to have a thickness of approximately 1-50 xcexcm. Preferably a temperature of about 800-1300xc2x0 C. is applied during the annealing processing step. In particular the method includes the steps of epitaxially growing a multilayer structure which consists of HTS films and insulating layers in an alternating manner on the smooth insulating layer.
A method of producing a multilayer structure comprising HTS films and insulating films is also provided which comprises the steps of providing a substrate structure as referred to above and epitaxially growing HTS films and insulating layers on the smooth insulating layer. Still further, the method advantageously includes the steps of etching out electronic circuits of the multilayer structure.
It is an advantage of the invention that the top insulating layer will be smooth enough for multilayers to grow epitaxially or top of it and that the structure can be made in a comparatively simply and cheap manner and thus that one of the main obstacles to the development of complex superconducting digital circuits and a microwave circuits is removed.